This invention relates to telephone system common equipment which is used to connect a local customer system to a central office of the telephone company and, more particularly, to novel circuitry for the interface connection.
In the telephone system, customer-owned common equipment, (or customer premise equipment, as it is sometimes called) is used to connect a local customer-owned telephone network to lines or "trunks" which come from to a central office of the telephone company. The major types of common equipment in use today include Private Branch Exchange (PBX) systems, Key Service Unit (KSU) systems, or hybrid combinations of the two. PBX systems generally use "2500 set" telephones which have only the standard 12-button dialing keypad. The PBX system automatically selects an available trunk line when service is requested by the handset being lifted out of its cradle or taken "off-hook". Additional features are incorporated in PBX systems by depressing various combinations on the standard keypad. KSU systems are used with telephones which have auxiliary buttons or "key" switches in addition to the standard keypad. These switches may be used for directly selecting and accessing desired trunk lines, as well as for implementing special functions. In all these systems, trunk interface circuitry provides the interface between the local system and the lines of the central telephone office.
The interface circuitry must include a DC termination, which is switched in to signal the central office that service is requested. When the DC termination is switched in, a current loop is formed in the two wires of the line between the common equipment and the central office, and means at the central office which detect the presence of the current flow provide an available line and supply a dial tone. This is a typical sequence which occurs in the standard "loop start" operating mode commonly used by KSU Systems.
PBX Systems generally utilize a different signalling mode between the central office and the common equipment called "ground start". In this mode, when service is requested by a subscriber telephone going "off-hook", one wire of the 2 wire trunk line is grounded. The central office responds by grounding the other wire of the line (if that line is available), and a loop current is thereby established, as before. Similarly, when the central office is transmitting an incoming call to the common equipment, it will ground one end of the line, and wait for the common equipment to acknowledge that the subscriber line is available by grounding the other end, thereby closing the loop current path. This "handshake" ground signalling procedure ensures that an outgoing phone call is not inadvertently connected with an incoming call on the same line, as can sometimes happen with a "loop start" configuration. Trunk interface circuitry designed for universal operation should provide for both "loop start" and "ground start" configurations.
The interface circuitry must also incorporate a detector to monitor the presence of the incoming "ring" signal as well as the presence, and preferably the value, of the DC line current. The value of DC line current established when the current loop is closed will vary significantly depending upon the distance between the common equipment and the central office, due to the resistance along the length of the line. Transmission gain equalization may be introduced in the system to compensate for the different lengths of the central office lines.
Still another function of the interface circuit is to provide a two-wire to four wire conversion, commonly known as a hybrid, to separate the bidirectional voice signals which are generally provided by the central office into two unidirectional signals which are utilized internally in the common equipment and in the individual telephone stations which the common equipment services. The four-wire unidirectional signals enable the use of voice band digitizing CODEC (i.e. coder plus decoder) equipment as well as other advanced digital electronics which are unidirectional in nature. It is also desirable for the trunk interface circuitry to allow for dial-pulse operation of older rotary dial telephones which may be in use at the customer's location and to support general supervisory functions to monitor the state of the overall system.
A subscriber telephone is connected through the local common equipment to a central office that contains switching equipment, signalling equipment and batteries that supply direct current to operate the telephone line. A DC termination is necessary for each line in order to control the current which flows in the line. The central office generally provides a line consisting of a two-wire connection. One of the wires is called TIP (or "T") and the other is called RING (or "R"). This terminology refers to the tip and ring parts of the plug which was used to make connections in manual switchboards. The switches in the central office respond to the presence of current flow in the line to establish a connection between the calling phone and the called phone. When the connection is established, the two telephones communicate over transformer coupled loops using the current supplied by the central office batteries.
A battery of approximately 48 volts is connected between the TIP and RING wires at the central office, with the TIP usually being connected to the positive side of the battery. As an example, when a telephone is in the idle, or "on-hook" condition, the DC circuit between the telephone and the central office is open and no current flows. However, when the telephone is placed "off-hook", the loop circuit to the central office is closed, and battery voltage at the central office causes current to flow. The current flow signals the central office to provide a dial tone to the line when it is ready to place the line in service.
A DC termination at the local end of the loop is required to limit the current flow to levels which are acceptable by the central office. The loop remains closed as long as the line is in use. The termination, in addition to limiting current, must therefore also provide a high impedance to the audio signals which are present on the line during conversation. To meet this requirement, an inductive element is desirable. In prior art designs, the transformer coupling network in the hybrid section has been used to additionally provide the necessary DC termination (typically 400 ohms). However, since a current of up to 120 milliamps will flow through the transformer winding, the transformer must be of sufficient size to support this high current without magnetic field saturation. The requirement for large transformers not only makes the equipment bulky, but leads to an increase in cost. Other prior art systems have marginally improved the design by incorporating a separate inductive element in front of the hybrid transformer to provide the proper termination path, and a capacitor to block the DC from the hybrid transformer. Although this allows the use of smaller hybrid transformers, the problem of saturation is now shifted to the single inductor element, and the inductor must again be suitably sized to avoid saturation.
Furthermore, these prior art designs provide a DC resistance path which is linear in character. The total resistance of the current loop is determined not only by the DC termination resistance, but also in large measure by the length of the current loop. This, of course, depends on the distance between the local subscriber equipment and the central office. In order to insure that a minimally acceptable current level flows through the loop for the case where the central office and subscriber are far away, a fixed resistance termination necessarily results in high current levels when the central office and the subscriber equipment are near each other. This results in undesirable increased power dissipation at the termination which can adversely affect adjacent circuitry in the trunk interface.